1. Field of the Invention;
The present invention relates to an electronic weighing apparatus. More specifically, the present invention relates to an electronic weighing apparatus wherein an analog voltage provided by a load converter such as a strain-gage type load cell is converted into a digital value, whereby the magnitude of the load is displayed in a digital manner.
2. Description of the Prior Art
Conventionally a strain-gage type load cell has been used as a load converter. One example of an electronic weighing apparatus employing a conventional strain-gage type load cell is shown in FIG. 1. Referring to FIG. 1, a weighing pan, not shown, for placing an article being weighed is coupled to a strain-gage type load cell 1. The load cell 1 comprises an energizing voltage source 3, a fine adjustment variable resistor 5 and a rough adjustment variable resistor 7. As is well known, such load cell 1 provides an analog signal of the magnitude which is proportional to the weight of an article placed on the weighing pan and the analog signal is applied to an amplifier 9. The amplified analog voltage provided by the amplifier 9 is applied to an analog/digital converter 11. The analog/digital converter 11 may comprise an integrated circuit "ICL8052/ICL71C03" manufactured by Intersil Incorporated, U.S.A. for example. The analog/digital converter 11 serves to convert a given analog voltage into a corresponding digital signal, which is applied to a display control 13 through a terminal T1. The display control 13 is responsive to a signal representing the sign of plus or minus of the given weight value provided by the terminal T2 as well as the given digital signal from the terminal T1 of the analog/digital converter 11 to cause a well-known digital display 15 to display the weight value.
In such conventional electronic weighing apparatus, it has been a common practice to make sure that the zero point of the load cell 1 and the display zero point in the display 15 coincide with each other. Accordingly, a fine adjustment is required for that purpose and such fine adjustment is accomplished by a variable resistor 5. Such variable resistor is liable to exhibit a poor temperature coefficient characteristic and is hence not necessarily stable with respect to a temperature variation. Accordingly, fine adjustment is required from time to time in accordance with a variation of an ambient temperature and it is difficult to display accurate weight data. Furthermore, the digital output obtained from the analog/digital converter 11 exhibits a stepwise variation, as shown in FIG. 2, for example, which requres that no output change occurs just at the zero point. More specifically, although a conventional weighing apparatus requires zero point adjustment to preclude an output change at the zero point, as shown in FIG. 2, the higher the resolution the more difficult is such zero point adjustment. Accordingly, a conventional electronic weighing apparatus as shown in FIG. 1 has made it impossible to enhance the resolution of an analog/digital converter.
In order to avoid the above described problem, it has been proposed that an offset value is in advance set so that the zero point of a load cell and the zero point of a display may deviate from each other, thereby to simplify a manual operation of an electronic weighing apparatus. As such an offset value, a value of approximately five percent of the full range of the scale, for example, is selected and such offset value is set in response to initiation of an operation of the apparatus upon turning on of a power supply, for example. On the occasion of weight measurement, the offset value, which is preset in advance or stored, is used for subtraction from an analog/digital converter output and the difference value is displayed as a weighed value. Thus, an electronic weighing apparatus of a type for setting an offset value avoids precise fine adjustment and accordingly can simplify a manufacturing process of the apparatus. More specifically, by setting such an offset value, any influence on a display exerted by a variation of the weight value of a placing pan for placement of an article is avoided, if such variation of the weight of a placing pan is within the range of the offset value. Hence a fine adjustment becomes unnecessary. Furthermore, since such weighing pan can be exchanged with relative freedom without precise adjustment, maintenance of the apparatus is also facilitated.
Thus, in an electronic weighing apparatus of a type for setting an offset value, such an offset value is stored in a memory. The timing for storing such offset value in a memory is selected to be responsive to initiation of an operation of the apparatus, so that the output of an analog/digital converter may be stored in a memory after a predetermined time period, say five to ten seconds, following turning on of a power supply, for example. Thus, the apparatus is structured such that the content in the memory is again applied to the output of an analog/digital converter each time when the power supply is turned on without regard to the magnitude of the weighed value at that time. In other words, even if the output from the analog/digital converter at the time of storing an offset value in a memory is relatively large, such value is stored in the memory as an offset value.
On the other hand, a load cell for use in such an electronic weighing apparatus and a structure for supporting such load cell are not necessarily designed to have a surplus mechanical strength. Accordingly, the output from such load cell exhibits a linearity characteristic only within a given limited range. Therefore, the linear portion of the output from an analog/digital converter receiving such output from a load cell is also limited to a certain restricted range. In other words, although the output of a load cell and thus the output from an analog/digital converter exhibits a linear change characteristic with respect to a variation of the weight in a given predetermined range which is inherent in an apparatus, the output does not necessarily exhibit such a linear change when the output exceeds such range. Accordingly, if and when an offset value being set in advance immediately before a weighing operation is too large and a weight value of an article being weighed is added thereto, the sum value could exceed the above described linearity range. In such a case, the weighing accuracy may not necessarily be satisfactory.
Meanwhile, in such apparatus for setting an offset value described above, it has been a common practice that a display is controlled to display "0" until such offset value is set. Furthermore, in a case where an offset value is stored in a memory in response to turning on of a power supply, the display is controlled to display "0" until after the lapse of a predetermined time period and therefore an operator cannot check the range or the magnitude of the offset value. In other words, in such a conventional apparatus, an operator cannot observe the magnitude of an offset value being stored in a memory when the weighed value is displayed. Accordingly, it is a disadvantage that the weight measurement is made without knowing whether a large change of such an offset value has occurred which would reduce the weighing accuracy.
In an apparatus in which an offset value is not stored in a memory in response with turning on of a power supply but an offset value is renewed manually, such offset value can be checked by looking at a display. However, when an apparatus is structured such that an offset value may be manually set in a memory, an offset value is renewed after an operator himself checks the magnitude of such offset value. Accordingly, in the case where such offset value is set through manual operation, it may occur, unless the range of an offset value inherent in the apparatus is known, that an erroneous judgement and accordingly an erroneous offset value renewal is made. Such a situation also reduces the operating facility of such an apparatus and deteriorates the weighing precision of the apparatus.